FIG. 1 is a simplified perspective or isometric view of a portion of a conventional “microstrip” transmission line 10. In FIG. 1, the structure 10 includes a planar dielectric plate 12. An elongated “strip” electrical conductor 14 extends over the upper surface 12us of the dielectric plate 12, and an electrically conductive “ground plane” 16 extends over the entirety of the lower surface 12ls, at least in the region generally under the strip conductor 14. Structure 10, and other generally similar structures such as “stripline,” tend to constrain the electrical fields associated with propagating electromagnetic waves to lie principally in a portion of the dielectric plate 12 lying between the strip conductor 14 and the ground plane 16, all as is well known in the art. In order to prevent excessive transmission perturbations or “losses” attributable to reflections of propagating electromagnetic energy, the “surge” or “characteristic” impedance of a transmission line, such as transmission line 10 of FIG. 1, must be maintained along its length, or at the very least must change “slowly” along its length, where the rate of change of characteristic impedance is in part dependent upon the wavelength. The type of transmission line illustrated in FIG. 1 is one of those commonly used in High Density Interconnect (HDI) technology, which is useful when making very compact, complex or repairable electronic systems.
FIG. 2a is a simplified cross-sectional representation of a prior-art arrangement using a microelectromechanical (MEMS) switch in conjunction with high density interconnect (HDI) structures. MEMS structures are mechanical structures made, in general, by processes which are akin to those used to fabricate solid-state integrated circuits, including photolithography and resist, etching, multiple layers of material. In FIG. 2a, a transmission line 10 includes a layer of dielectric 12, which has a strip conductor 14 on its upper surface, extending front a left end LE to near a transverse plane T6. A ground plane or conductor 16 extends from the left edge LE to near a transverse plane T2. At the right end RE of FIG. 2a, a similar transmission line 210 includes a dielectric slab 212 defining an upper surface 212us and a lower surface 212ls, and a strip conductor 214 overlying upper surface 212us from near a transverse plane T14 to right end RE. A ground plane 216 extends below, and in contact with, lower surface 212ls from the right end RE to transverse plane T18.
A MEMS switch structure designated generally as 220 lies under HDI interconnect transmission-line structures 10 and 210 in FIG. 2a. MEMS switch structure 220 includes a MEMS dielectric substrate 222 defining an upper surface 222us and a lower surface 222ls. The movable mechanical element in MEMS structure 220 is illustrated as an electrically conductive switch contact 224, which is fabricated so that a drive structure (not illustrated in FIG. 2a) can cause it to move upward and downward (relative to the orientation of the FIGURE) in the direction of double-headed arrow 250. In order to incorporate the movable element 224 into a transmission line, a further strip conductor 234 is deposited on or otherwise supported by the upper surface 222us of dielectric plate 222, extending partially under movable switch element 224, with a break 235 in the continuity of strip conductor 234 generally at the location of the movable element 224. When the movable element 224 is in its uppermost state or condition, which is the position illustrated in FIG. 2a, there is no continuity between the left and right portions of strip conductor 234, and the switch is therefore OPEN or nonconductive. Conversely, when the movable conductor element 224 is in its lowermost state or condition, it is in contact with both left and right halves or portions of strip conductor 234, and provides electrical continuity therebetween. In this state, the switch is said to be CLOSED. It should be noted in passing that European usage looks on a switch as one might a gate, and a nonconductive state is known as CLOSED, while the conductive state is known as OPEN. Movable switch element 224 is controlled to the UP or DOWN state by MEMS controllers, not illustrated.
In order to avoid transmission-line discontinuities which might perturb proper transmission, it is desirable to have strip conductor 234 of FIG. 2a in the form of a transmission line. The transmission line of MEMS structure 220 includes a further ground plane 226 lying below lower surface 222ls of MEMS substrate 222, at least in the region lying below strip conductor 234 and movable element 224.
In order to provide a space or “room” for the desired movement of movable conductive element 224 of the MEMS structure 220, a layer 240 of dielectric is placed between the transmission line structure 210 and the MEMS structure 220, with a gap or opening 242 at the location of movable element 224. Finally, the connections are completed by a plurality of through vias and metallizations. More particularly, a through via 250 extends at transverse plane T2 from ground plane 16 to a metallization 251, and a further through via 252 extends at a transverse plane T4 from metallization 251 to ground plane 226. Thus, the combination of through vias 250 and 252, in conjunction with metallization 251, provides contact between the right-most end of ground plane 16 and the left-most end of ground plane 226. In addition, a through via 256 extends at transverse plane T18 from ground plane 216 to a metallization 257, and a further through via 254 extends at a transverse plane T16 from metallization 257 to ground plane 226. Thus, the combination of through vias 254 and 256, in conjunction with metallization 257, provides contact between the left-most end of ground plane 216 and the right-most end of ground plane 226. Some strip conductor connections are made by means of a through via 260 extending at a plane T6 through dielectric plate 12 to a metallization 261, and a further through via 262 extending through dielectric plate 240 at plane T8 from metallization 261 to the left-most end of strip conductor 234. The strip conductor connections are completed by means of a through via 266 extending at a plane T14 through dielectric plate 212 to a metallization 267 lying between dielectric plates 212 and 240, and a further through via 254 extending at a plane T12 through dielectric plate 240 to the right-most end of strip conductor 234. Thus, through vias 264 and 266, in conjunction with metallization 267, provides electrical continuity from strip conductor 214 to the right end of strip conductor 234. In general, it may be said that the fields associated with a propagating electromagnetic wave are constrained to lie between the strip conductor/ground plane sets 14,16; 234, 226; 214, 216.
FIG. 2b illustrates the electric field resulting at transverse plane T1 of FIG. 2a from application of a direct voltage to strip conductor 14 relative to ground 16 of FIG. 1a. In FIG. 2b, the dielectric 12 is not hatched, in order to make the electric field lines 290 more visible. As illustrated, the electric field lines 290 extend from the strip conductor 14, principally through the dielectric material 12, and terminate on ground conductor or plane 16. FIG. 2c illustrates the electric field resulting at transverse plane T9 of FIG. 2a from application of a direct voltage to strip conductor 14 relative to ground 16 of FIG. 1a. As illustrated, the electric field structure 292 of FIG. 2c is virtually identical to that of FIG. 2b, with the field lines extending principally through the dielectric material 222 from the strip conductor 234 to the ground plane 226. The similarity of the field structure is an indication that the surge impedance of this section of transmission line is similar to that of the section illustrated in FIG. 2b. 